Currently, aircraft are limited in flight range and flight duration because of fuel capacity and fuel consumption. These limits require aircraft to return for refueling and also prevent them from unlimited flight ranges. Further, aircraft are also limited by weight constraints, such as the weight of the fuel necessary for travel that limits the speed and cargo capacity of the aircraft. Some airplanes use solar power to eliminate these issues, however solar power systems experience power reductions due to weather and atmospheric changes, the Earth's annual and diurnal cycles, or solar eclipses. Therefore, aircraft that use solar power require supplemental or alternate sources of energy. Laser or microwave power beaming from the Earth's surface, from other aircraft, or from satellites orbiting the Earth can be used to supplement the solar power to aircraft.
Current systems that provide laser-beamed power to aircraft by using photovoltaic (PV) arrays and receivers have problems with obtaining uniformity of power transfer across the PV array. A PV array designed for solar powered applications includes PV cells arranged on a flat planar surface. In solar powered spacecrafts or terrestrial solar power applications, the planar PV array experiences generally uniform, steady irradiance across all the cells in the array. However, the planar PV array does not experience a uniform irradiance across all the cells in the array when illuminated with laser-beamed power. The intensity of the laser varies across the width of the beam, with higher intensity at the center of the beam and generally weaker intensity away from the center. The irradiance received by a planar laser-power PV receiver is therefore non-uniform, with the strongest rays at the center of the PV array and weaker rays impinging upon the PV cells toward the edges of the PV array.
Currently, planar PV arrays or receivers that are designed for use with solar power have rows of identical photovoltaic cells arranged on a flat surface. With uniform illumination, all of the PV cells in the PV array receive the same irradiance of light and produce approximately the same output power. The PV cells are connected in series in a string, and the strings are then connected in parallel with other strings to form the PV array. The PV cells are connected in series to produce a sufficient voltage. Within a string, all of the PV cells conduct the same amount of current. Multiple strings are connected in parallel to produce adequate current at the selected voltage. When the PV array is illuminated with uniform solar rays, each string of PV cells produces the same current. Further, since every PV cell in the array has roughly the same internal resistance, the resistive loss in each cell is approximately the same.
Unlike sunlight, a long-range laser beam illuminating a planar PV array has a roughly Gaussian time-averaged intensity profile. The Gaussian average intensity profile results in varied irradiance across the PV cells. The PV cells at the center of the PV array have much higher irradiance than the PV cells at the edge of the array. If a PV array is designed for optimum efficiency at the lower irradiance levels at the edges, the PV array will likely experience overheating at the center of the array, because the PV cells in the center are not rated for the higher irradiance beams incident on the center. The overheating of the PV cells at the center increases their electrical resistance and therefore increases resistive losses for the array. Conversely, a PV array designed for optimum efficiency at the high irradiance in the center of the PV array results in reduced average power production per PV cell, i.e. the PV cells at the edge do not receive the total irradiance for which they are rated, resulting in reduced power output. In either case, costly resources, i.e. PV cells, are used below their optimum capacity. Therefore, a system and method is needed for achieving uniform irradiance on all PV cells in a laser-powered PV array.
Another deficiency in current planar PV arrays systems is that atmospheric inhomogeneities create rapid variations in refractivity across the width of the laser beam. These variations in refractivity cause variations in irradiance across the surface of the PV array. Short-term variations in irradiance caused by inhomogeneities of the atmosphere affect the current produced by each cell in the array. Greater current from one cell increases the I2 R losses of all cells in the string and results in greater overall average losses for the entire array. Therefore, a system and method of adjusting for the variations in irradiance across the array cells is needed.
Thus, in view of the disadvantages of the current methods and systems, and in view of the current demand in the relevant art, what is needed is a method and system to provide PV laser-beamed power to satellites and aircraft while mitigating the effects of the current unsteady, non-uniform illumination in these systems.